Heating, ventilation and air conditioning assemblies in automobiles typically include a large, molded plastic housing, often called an HVAC housing, which serves to duct and control forced air flow, as well as to contain various heat exchangers and controls therefor. Three different control systems interact to control air flow through the housing and the temperature thereof. Furthest upstream, a fresh air/recirculated air control determines whether a blower pulls in outside air, or recirculates internal air, or some combination thereof. A greater proportion of recirculated air speeds up the heating or cooling process. Furthest downstream, a so called mode control determines whether forced air is ultimately directed through air outlets upwardly at the windshield, straight forward at the occupant, downward at the feet, or some combination thereof. Between the air inlet and outlet, a pair of separate heat exchangers contained within the housing serve to temper the drawn in air, cooling it, heating it, or both, as determined by a separate temperature control. Typically, all three sets of controls have been simple flapper doors which rotate back and forth to open up or close off various passages and openings, or to direct air flow in one direction or the other, or both. Recently, designs have been proposed which replace the flapper door with a mechanism similar to a window shade, generally called a film valve, that shifts back and forth to cover or uncover various openings and passages. This presents obvious advantages of compactness, and also allows a more finely adjustable opening and closing. Replacement of the downstream air inlet control and of the upstream air outlet control with film valves is fairly straightforward. The intermediate control system used to determine the air temperature presents unique challenges, however, because of the way in which the air flow must be controlled through and around the two heat exchangers involved.
Air temperature is controlled and determined with two heat exchangers carried within the housing, including an evaporator core, which can be turned on and off along with the rest of the air conditioning system, and a heater core, which is generally always activated and hot. The heater core traditionally has diverted engine coolant flowing through it whenever the engine is running, since heater core shut off valves represent an added expense, and also since the heater core can in fact be used in the summer to partially reheat refrigerated air that would otherwise be too cold for the desired interior temperature. The evaporator core typically is large enough in area to fill the entire cross sectional area internal to the housing, and is located downstream of the heater core. Therefore, all forced air passes through the evaporator core first, regardless of whether the evaporator core is activated. This is not a drawback, since the evaporator can be turned off when it is not desired to cool the air. Furthermore, having the evaporator core permanently sitting in the air flow path can present a benefit, even in winter, since it allows outside air to be cooled and dried before being heated, which is useful when defrosting the windshield. On the other hand, having the heater core always active can present a drawback. Known air temperature control systems do not completely isolate or insulate the air flow from the heater core, even when it is not desired to heat the air at all, as when the system is set for rapid cooling. Both older, flapper door temperature valves, and newer, proposed film type temperature valves controls, leave the downstream face of the heater core exposed to the cooled air flow leaving the evaporator core. This is generally true even when the cooled air flow is blocked from flowing directly through the heater core. Air can still flow or "scrub" across the exposed downstream face of the core, picking up some significant heat, when it would be preferable that it pick up little or none.
A good example of a conventional arrangement of evaporator core, heater core, and temperature door can be seen in U.S. Pat. No. 4,978,061, in FIG. 1 of that patent. An upstream evaporator core fills the whole cross sectional area of the housing, while a downstream heater core fills only about half, leaving a bypass passage around the heater core. A flapper door between the two cores moves from a down position, for full cold, to an up position, for full heat, and can also take up intermediate positions to divide the air flow up partially through, and partially around the heater core. In the down, full cold position, the upstream face only of the heater core is covered, and the bypass passage is left fully open. This is enough to block direct air flow through the heater core, but does not cover the downstream heater core face or prevent some of the bypassed air flow from swirling down and "scrubbing" across the exposed heater core back face to pick up some heat. Furthermore, as the door rotates between intermediate angular positions, it is not as precise or "linear" in its division of air flow between the heater core and the bypass passage as would be ideal for the best temperature control. The room needed for a door to swing in an arc also occupies a good deal of housing volume.
Proposed film valve designs for air temperature modulation have the potential for improved compactness and more precise air flow division, but still leave the downstream face of the heater core undesirably exposed to air flow. Examples may be seen in U.S. Pat. Nos. 5,326,316 and 5,162,020. Each uses a film valve to progressively cover or uncover the front face of the heater core, thereby determining how much air flows directly through it. The design disclosed in U.S. Pat. No. 5,564,979 does not use the film belt itself to directly block or unblock the bypass passage around the heater core, using a separate flapper type valve located upstream of the heater core instead. U.S. Pat. No. 5,162,020 does use the film valve to directly open and close the bypass passage, but basically replicates a conventional flapper door by pivoting one edge of a film sheet in front of the heater core and sliding its other edge back and forth, so that the film becomes a traveling leg of a triangle, in effect, moving over approximately the same pattern that a flapper door would follow. Again, neither design blocks the rear face of the heater core. Essentially the converse is disclosed in another patent, U.S. Pat. No. 5,154,223, where a continuous belt is located in front of the downstream face of the heater core, between the heater core and the air outlets, rather than behind the upstream face of the heater core. The heater core sits in a mid point position in the case, defining two bypass passages around the heater core. The one belt does double duty as a temperature control and air outlet control, acting both to progressively block off the downstream face of the heater core, while concurrently opening or closing the bypass passages and the various air outlets. Here, in so called full cold mode, it is the downstream face of the core that is blocked to prevent direct air flow through the heater core, while the bypass passages around the heater core are opened. However, the front face of the heater core instead is exposed straight on to the forced air flow. The only way to prevent the air directly impacting the heater core front face from being substantially heated would be to shut the heater core off with a separate coolant flow shut off valve.